It's really more about "TV" logic than anything--they wanted, and had budget for, a scene with the Dothraki Horde riding into battle. But TV logic also meant that Dany burned up 1000 wagons with supplies that would have been critical to her armies. That said...
In in-universe answer must be based on military strategy, as the questioner suggests:
- She wanted to occupy the territory. You need ground forces for that.
For a parallel, you can take the modern USA. This country can dominate the air in many regions around the world and impose "no fly zones" at will, but that doesn't seem to help all that much in Afghanistan b/c occupying territory ultimately requires infantry. Cavalry (in modern warfare, tanks) can be extremely effective as a component of "shock and awe" strategies.
It's confusing in that Dany burned up the supply wagons, but ostensibly, there are more supplies to be had in the Reach.
It may also be that she is occupying territory around King's Landing to put pressure on Cersei.
- In the case of an air-only attack, the ground forces can scatter and seek cover
This situation is distinct from Dany's earlier proposal of tracking down and burning Euron's fleet, in that there is no cover for ships, it takes them longer to scatter, and the number of targets is reasonable.
By contrast, there are likely thousands of soldiers, and tracking down individuals after they scatter would not be feasible. Once the Dany flies back to Dragonstone, the troops could regroup.
We won't have a definitive, hole-free answer until Martin publishes the next book though.
Addendum based on subsequent episode:
- Dany used the securing of the territory as a way to convert or eliminate an enemies
Randyl Tarly refused to bend the knee and was burned to ashes. The remaining enemy combatants all bent the knee.